One of the biggest challenges in current SMR plant design is the heat transfer process from the inside tube wall to the bulk gas The situation can arise whereby improving the geometric surface area (GSA) without improving the packed bed heat transfer coefficient (HTC) means that although there is enhanced catalyst potential, it cannot be realized, since the heat transfer mechanism is inadequate.
It is the thin layer of gas close to the tube wall which dominates the heat transfer process, and therefore the HTC is determined by conditions close to the wall, rather than the bulk properties of the catalyst bed. Typical correlations take the form:HTC=C′d−0.25ew−2 where:    C′=constant    d=equivalent sphere diameter    ew=voidage near wall
The HTC at the wall is determined by the particle equivalent sphere diameter (d), as voidage approaches unity in this region. Therefore to improve the HTC, catalyst pellets with smaller equivalent sphere diameters should be used. These have the effect of more pellets lying closer to the wall breaking the film, thus reducing the resistance to heat transfer.
If a catalyst with increased activity is used, then a smaller catalyst volume is required, increasing the space velocity in the tube. This increases the HTC as more turbulent conditions are created. Improving the HTC has a significant effect on the reformer performance.
During the heating of the reformer, a gap may be created between the reformer tube wall and the porous media cylinder, due to differential thermal expansion, which creates a preferential path for the flow of the feed gas along the inner surface of the SMR tube. The gap would be most prominent for a ceramic support material (significant difference in thermal expansion factor of alloy SMR tube and ceramic support of SMR catalyst).
One approach to improve the heat transfer is to use porous media with supported active phase (catalyst). This porous media may be ceramic or metallic in nature. Ceramic or metallic porous media cylinders with deposited catalyst are located in SMR tube. The proposed solution consists of wrapping the foam inserts in a shape memory alloy (SMA) sheet, mesh which will “deploy/expand” at elevated temperature to close the gap between the tube and porous media catalyst insert.